[MrForbes]

The problem, is that you are using wheels that are too large and grippy for the drivetrain you have connected to them. It's kind of late in the build season to fix this....but suggestions would be to see if you can adjust the height of one pair of end wheels to make it sit up higher than the others, or maybe replace one set of end wheels with something a little smaller, or with less traction.

The basic problem of having not enough gear reduction ratio in the transmissions is harder to fix. If you can get pulleys (or chain sprockets) with different sizes, then you could get more speed reduction (an torque increase) and it would help.

without pictures of your robot, to see how it's built, it's not easy to make useful suggestions.